The objective of this project is to determine the mechanism(s) whereby the pituitary promotes the development of Gross virus-induced leukemia. To this end two principal approaches are being taken. One, the effect of replacement hormone treatment of virus-infected hypophysectomized mice; and two, the characterization of selected hormone receptors in and on luekemic T-lymphocytes. At this point in the study it appears that, 1. Thymosin treatment will not influence the process of leukemogenesis in the mouse. 2. Leukemic T-lymphocytes possess amounts of glucocorticoid, histamine, and norepinephrine receptors comparable to that present in normal thymocytes. Our speculation at the moment is that the murine leukemia virus impairs the ability of thymic epithelial cells to induce the differentiation of thymocytes; a situation resulting in the propagation within the animal of a large population of lymphocytes unresponsive to the normal hormonal control mechanisms.